advice on pressure sensors

Hi all,

I'd much appreciate any advice on my search for a cheap and simple pressure sensor. I need to make a device which can measure how hard a baby is squeezing a silicon tube, around 2cm thick. I figure the best way is to connect the tube via a pneumatic tube to a pressure sensor. So what I need is a pressure sensor that does not have to be accurate in absolute terms, but needs to be relatively sensitive, so it can measure changes in grip strength. I reckon if it went up to 1000 kPa that would be more than enough. Preferably it would come with an integrated USB interface and drivers which provided an API. Alternatively, if you know a good one but it just outputs a voltage, perhaps you could also recommend a good cheap and cheerful USB A to D converter?

So far the best thing I've found is this, which you can combine with a USB interface from the same supplier:

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But I think it will only interface using their proprietary software - it's for classroom use, after all. But this gives you a basic idea of what I'm after.

Other solutions to the basic problem also gratefully received!

Cheers,

Ben

-- Dr. Ben Kenward Department of Psychology Uppsala University, Sweden

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Sounds reasonable to me. Another way to do it is probably with a strain gauge, but that might require that the baby hold something in a specific orientation, which isn't going to happen.

I wonder if it might be helpful to have a short section of relatively soft tube for the baby to squeeze, and then a longer section of stiffer tube linking the squeeze tube to the measurement device. This way, the pressure increase provided by the baby's squeeze isn't offset by the long tube to the sensor swelling up.

Maybe babies are stronger than I think, but that seems like a lot of pressure (145 psi). I just did an experiment in my garage with some automotive fuel line and a pressure gauge. The fuel line is braid- reinforced rubber hose, about 15 mm outside diameter and 8 mm inside diameter - about 3.5 mm wall thickness. I used a piece about 7 cm long, plugged one end of it, and connected the other to a pressure gauge. Even squeezing the hose with fingers from both hands, the best I could develop was about 5 to 6 psi (34 to 42 kPa) over atmospheric.

The silicone tubing that you propose is very likely much more flexible and easier to squeeze, and has a larger internal volume. However, I still think you won't get much pressure this way. If you assume that there is a 8 cm total length of tubing, it starts at atmospheric pressure, the baby can completely flatten the last 4 cm of it, and the remaining 4 cm don't swell up at all, then the pressure in the tube will be about twice atmospheric.

You might look at the current Digi-Key catalog, pages 1601 to 1610. Sensors that might do what you want with a 0-5 V output are around US$17 to $35 in quantity 1. I know that USB A/D boxes exist but I don't have a favorite. Depending on how fast you need to make readings, an alternative might be a digital multimeter that has RS-232 serial output, and a USB-to-serial adapter cable. This is probably only good for up to a few readings a second, though.

Matt Roberds

PS I very much enjoyed reading about the results of the crow experiments back when they came out.